you need to talk to your international adviser and tell the reason your i-20 got expired. as a matter of fact, your international admissions office should have sent you a mail or e-mail at least a month before it got expired. it could be possible you didn't notice that. also, you can still reinstate and the probability of reinstating is lot better if you've beein maintaining your status by taking minimum 12 credits, good GPA etc. Also have good reasons for i-20 expiration. you shouldn't worry if you meet all these requirements. your international adviser should help you out with the process. you also, need to write a letter stating the reason behind your i-20 expiration when you reinstate.